This invention relates to a process for forming shaped pultruded composites.
Pultrusion is a known process by which composites of longitudinally oriented reinforcing fibers in a resinous matrix can be prepared. The process generally involves pulling a plurality of reinforcing fibers through an impregnation area to coat them with resin. The coated fibers are then drawn through a consolidation unit and a die, in which the coated fibers are packed tightly together and formed into the desired cross-sectional shape.
The pultrusion process is primarily used with thermoset resins, and in that case the die is heated in order to cure the resin as the fibers are being consolidated and shaped. More recently, the pultrusion process has been applied to make composites using certain depolymerizable and repolymerizable thermoplastics, especially thermoplastic polyurethanes, as described in U.S. Pat. No. 5,891,560.
A limitation of the pultrusion process is that it is adapted only to produce straight, elongated parts of uniform cross-section. This is due in part to the nature of the process itself, and in part due to the thermoset resins that are most often used. Because the process operates by pulling long, continuous reinforcing fibers through the various unit operations, it naturally forms elongated composites in which the reinforcing fibers are aligned in the direction of the pultrusion. Once cured, thermoset resins cannot be subsequently thermoformed or shaped in any other manner, yet it is necessary to cure the thermoset resin at the die in order to bind the fibers together and create the desired cross-sectional shape.
In U.S. Pat. No. 5,798,067 to Long, a process is described whereby a shaped pultruded composite is made. In this process, a roving consisting of a plurality of fibers in a partially cured thermoset resin is formed. This roving is then passed through a pair of heated, matched rotary dies that simultaneously impart the desired shape to the roving and cure the thermoset resin. In actual practice, however, this process is difficult to manage. The cure of the thermoset resin must be very carefully managed so that the resin can hold the individual fibers in the roving together until the final cure is complete, yet permit the roving to be shaped by the rotary dies. It is not practical to achieve this on a commercial scale.
Thus, it would be desirable to provide a pultrusion process that is capable of producing profiles incorporating regular, predictable curvatures or torsional twists.